1. Field of the Invention
The present invention relates to a gas sensor, and more particularly, to a porous protective layer for a gas sensor, which is formed on a surface of a gas sensor, a method of manufacturing the same, and a gas sensor including the porous protective layer.
2. Discussion of Related Art
Due to air pollution, environmental pollution, and stability issues of industrial fields, which are caused by industrial development, the necessity of gas sensors for sensing various harmful environmental gas species (e.g., H2S, H2, CO, nitrogen oxides (NOx), sulfur oxides (SOx), NH3, volatile organic compounds (VOCs), etc.) is increasing. Among the gas species, H2S is a gas contained in bad smell such as bad smell, bad breath, and the like, and should necessarily be measured to purify an environment and establish a pleasant living environment. Also, CO is a gas that is contained in an exhaust gas of gasoline vehicles, sooty smoke discharged from industrial fields, or the like, and should be measured to prevent environmental pollution and maintain a pleasant interior environment of vehicles by adjusting an amount of emission of vehicles driving on roads and adjusting CO gas inflow into an interior of a following vehicle. Specifically, gas sensors capable of measuring oxygen, hydrogen, carbon monoxide, carbon dioxide, NOx, and the like are widely used for detecting a specific gas component in an exhaust gas or measuring the concentration thereof in an internal-combustion engine or the like.
Generally, a porous protective layer is formed on a surface of a gas sensor. The porous protective layer may protect the gas sensor not only from the above-described liquid materials and poisonous materials but also from external impacts, and may be made of a porous material so that the gas sensor may sense a measuring gas. In Japanese Registration Patent No. 4691095 in the related art, a sensor element for measuring physical characteristics of a measuring gas is disclosed.
However, in the related art, there is a problem in that a porous protective layer formed on a gas sensor does not completely block liquid materials, cracks are generated in an external sensing electrode due to thermal shock caused by a temperature difference between the liquid materials and the gas sensor, or materials other than a test gas to be inspected also directly reach the gas sensor due to separation of the porous protective layer from the gas sensor, cracks, or pore collapse. Also, there is a problem in that poisonous materials are easily deposited on the porous protective layer, a response speed of the gas sensor to a measuring gas is lowered, and a concentration of the measuring gas may not be accurately measured.